It doesn't seem to activate while wearing anything short of the full set, whereas the UI states it should be activating at 2 pieces. I've now tested in natural combat and by shooting the training dummies until I crit while wearing the full set, at which point I see the buff icon appear above my health bar and the matching increase on my primary weapon damage in the detailed inventory menu. Trying this at 2/3/4 pieces grants no buff.
What's more, I can't even determine that its 5-piece set perk (bonus increase to speed) functions properly. The speed on my primary weapon stays the same regardless of how much mentor gear I'm wearing and the same applies for huldufolk (bonus increase to speed and armor, neither of which show a change in value in the inventory menu).